1. Field of the Invention
The present invention relates to superconductor filter units used for a radio base station and the like.
2. Description of the Related Art
In recent years, with rapid development of radio communications, a high speed and large capacity transmission technology has become indispensable, and the expectations for a superconductor filter device using a high temperature superconductor are increasing. Superconductor has an extremely small surface resistance also in a high frequency region as compared with an ordinary electrically good conductor. This allows the transmission loss to be kept low even if the superconductor filter devices are multi-staged. Accordingly, the superconductor filter device allows excellent frequency cutoff characteristic to be obtained and allows frequency resources to be utilized effectively. However, in order to actually operate the superconductor filter device, the superconductor filter device needs to be cooled to ultra low temperature of the order of 70K. Namely, since the electric resistance of high temperature superconductor is high at room temperature, the superconductor filter device needs to be cooled. Then, in the conventional superconductor filter unit, a superconductor filter device and a cooler cooling the same are housed in a vacuum housing.
FIG. 10 is a view showing a conventional superconductor filter unit. In the conventional superconductor filter unit, on a package base 101 is disposed a superconductor filter device, which is covered with a lid 102. A metal package is composed of the package base 101 and the lid 102. A cooler 105 cooling the superconductor filter is provided under the package base 101 and these are housed in a vacuum housing 106. Moreover, on the exterior of the package base 101 is mounted a connector 107, to which is connected a semi-rigid coaxial cable 103 whose both ends are provided with connectors 104.
FIG. 11 is a view showing the interior of the conventional metal package. On the package base 101, a dielectric substrate 111 is provided via a grounding electrode and a superconductor film. On the dielectric substrate 111, a plurality of resonators 112 is arranged, the resonator being made of high temperatures superconductor and patterned in a hairpin shape. The resonators 112 is coupled to each other and thus constitutes a plane circuit type filter device. Moreover, the resonator 112 at the end is connected to a signal input/output line 131 via an electrode 114 and a solder material 115. This signal input/output line 131 is connected to the connector 107 for a coaxial cable. The input/output of a signal is performed via the signal input/output line 131 and the semi-rigid coaxial cable 103.
In such a superconductor filter unit, the frequency cutoff characteristic of the filter can be made abrupt by increasing the number of resonators 112, i.e., by multi-staging. Moreover, the plane circuit type filter is shielded from external high frequency signals by the package base 101 and the lid 102.
The superconductor filter unit is used in a radio base station and the like and is disposed, for example, directly under the antenna at the top of a steel tower of the base station, or the like. For this reason, in view of the transporting work and installation work, and the like, the superconductor filter unit is preferably miniaturized as much as possible. However, in the conventional superconductor filter unit, it is difficult to prevent the inflow of heat from the outside, and therefore the cooler 105 needs to be upsized to the extent to meet this need. Accordingly, the miniaturization of the superconductor filter unit itself has limitations.
Patent Document 1 (International Publication No. WO 00/52782) discloses a technique in which treatment is applied to the coaxial cable itself for the purpose of suppressing the inflow of heat via the coaxial cable. This technique may attain an intended purpose but may not satisfactorily miniaturize the superconductor filter unit.